I had a form open in the GUI Designer. I typed cmd-F and nothing happened. Oops. A lower case f showed up at left of I think the outermost outline in the GUI designer. I tried to undo. Then the entire builder panel went light gray and has a single message in it (which I have to type in here by hand, unfortunately): Form file is invalid: Error on line 204 of document file://blahblah/Selena-6913.app/bin/UTF-8: The element type children must be terminat... Attached are before and after excerpts of the form file, where you can see how bizarrely mangled the xml is.